Looking at the schematic, it appears that if you put a headphone plug into the Pokitto’s headphone jack, then you could safely feed an audio signal into the PEX pin. The plug could be connected to headphones or just be an empty, standalone one with all connections left open, since its purpose is only to open up the switch in the headphone jack.
Doing this, the audio would come out of the speaker. You wouldn’t be able to send audio from the PEX to headphones plugged into the jack. The volume of the audio coming out of the speaker would be strictly controlled by the amplitude of the of the PEX signal (which should not exceed 3.3V). This is because Pokitto’s on board volume control circuitry will not be in the signal path.